Lubrication fluids are applied between moving surfaces to reduce the friction between such surfaces, thereby improving efficiency and reducing wear. Lubrication fluids also often function to dissipate the heat generated by the moving surfaces.
One type of lubrication fluid is a petroleum-based lubrication oil used for internal combustion engines. Lubrication oils contain additives which help the lubrication oil to have a certain viscosity at a given temperature. In general, the viscosity of lubrication oils and fluids are inversely dependent upon temperature. When the temperature of lubrication fluids is increased, the viscosity of such fluids generally decreases, and when the temperature is decreased, the viscosity of such fluids generally increases. For internal combustion engines, for example, it is desirable to have lower viscosity at low temperatures to facilitate engine starting during cold weather, and a higher viscosity at higher ambient temperatures when lubrication properties typically decline.
Such additives for lubrication fluids and oils include rheology modifiers, including viscosity index (VI) improvers. VI improving components, derived from ethylene-alpha-olefin copolymers, modify the rheological behavior to increase the lubricant viscosity, and promote a more constant viscosity over the range of temperatures over which the lubricant is used. Higher ethylene content copolymers efficiently promote oil thickening and shear stability. However, higher ethylene content copolymers tend to flocculate or aggregate from oil formulations. This typically happens at ambient or subambient conditions of controlled and quiescent cooling. This deleterious property of otherwise advantageous higher ethylene content viscosity improvers is measured by low temperature solution rheology. Various remedies have been proposed for these higher ethylene content copolymer formulations to overcome or mitigate this propensity towards the formation of high viscosity flocculated materials.
Conventional vanadium-based Ziegler-Natta catalysts are typically most useful in polymerizing copolymers composed of ethylene and propylene only. While copolymers of ethylene and higher alpha-olefins, such as butene, may be produced, such copolymers are limited to those having higher ethylene content.
Metallocene-based catalysts may be used to produce higher-alpha olefin content in VI improvers, as noted in U.S. Pat. Nos. 6,525,007 and 5,446,221, which are incorporated herein by reference.
The performance of VI improvers can be substantially improved, as measured by the thickening efficiency (TE) and the shear stability index (SSI), by appropriate and careful manipulation of the structure of the VI improver. We have discovered that such performance improves when the distribution of the monomers and the chain architecture are controlled and segregated into at least two compositionally disperse and/or crystallinity disperse polymeric populations. These disperse polymeric populations may be achieved by the use of a special synthesis process that employ metallocene-based catalysts in the polymerization process.
Metallocene-based catalysts used in continuous feed stirred tank reactor lead to ethylene copolymers which are compositionally narrow and have a most probable narrow distribution in molecular weight. Such a concomitant distribution of molecular weight and composition would be characterized as a discrete component of the VI improver.
One solution proposed is the use of blends of amorphous and semi-crystalline ethylene copolymers for lubricant oil formulations. The combination of two such ethylene-propylene copolymers allows for increased TE, SSI, low temperature viscosity performance and pour point. See, e.g., U.S. Pat. Nos. 7,402,235 and 5,391,617, and European Patent No. 0 638,611, the disclosures of which are incorporated herein by reference.
We have found that, contrary to the teachings in the art, there is a preferred relationship between the amount, composition and molecular weight of the discrete distributions of the ethylene-based alpha-olefin copolymers used in the polymeric blends for VI improvers. This relationship leads to ethylene-based alpha-olefin copolymers which have a controlled population of monomers such that it has a superior performance in the TE at a predetermined SSI. The choice of the alpha-olefin (e.g., propylene or butene) will affect other properties of the rheology modifier such as solubility parameter, TE and SSI. It is believed that the addition of alpha-olefins may in addition lead to a further degree of control in the polymer chain such that the level of crystallinity will be diminished and thus the fluidity of the solutions containing the polymers will be enhanced.
There remains a need, however, for novel rheology modifier compositions comprised of ethylene and alpha-olefin-based comonomers suitable for use in VI improvers which have unexpectedly high TE as compared to the prior compositions while still being equivalent in their beneficial low temperature solution rheology properties. This invention meets this and other needs.